Semiconductors and diodes
Semiconductors made electronics possible. By controlling where current can flow, diodes rectify, regulate, and emit light.
Semiconductors
Semiconductors, chiefly silicon, conduct better than insulators but worse than metals, and their conductivity can be controlled precisely. Pure silicon has few free charge carriers. Doping adds impurities: phosphorus creates n-type silicon with extra free electrons, and boron creates p-type silicon with 'holes', missing electrons that behave like positive carriers.
The p-n junction
Where p-type and n-type regions meet, electrons and holes diffuse across and recombine, leaving a depletion region with no free carriers and a built-in electric field. Applying forward bias, p side positive, overcomes the barrier and current flows easily. Reverse bias widens the depletion region, and only a tiny leakage current flows. This one-way behaviour defines a diode.
Diode characteristics
A silicon diode conducts noticeably above a forward voltage of about 0.6 to 0.7 V; the current rises exponentially with voltage, as described by the Shockley equation \(i = I_S(e^{v/(nV_T)} - 1)\), where \(V_T\) is about 25 mV at room temperature. In simple analysis, a conducting silicon diode is modelled as a 0.7 V drop. Schottky diodes have lower forward drops, about 0.2 to 0.4 V.
Rectifiers
Rectifiers convert AC to DC. A half-wave rectifier uses one diode to pass only positive half-cycles. A full-wave bridge rectifier uses four diodes so both half-cycles appear with the same polarity. Adding a large smoothing capacitor holds the voltage near the peak between cycles, leaving a small ripple. This is the front end of countless power supplies.
Zener diodes and regulation
Zener diodes are designed to conduct in reverse at a well-defined breakdown voltage, such as 5.1 V, without damage. With a series resistor, a Zener diode holds a roughly constant voltage across a load despite changes in supply voltage, forming a simple voltage regulator or reference. Integrated regulators such as the classic 7805 provide better regulation.
LEDs and photodiodes
Light-emitting diodes emit light when forward current flows, as electrons and holes recombine; the semiconductor material sets the colour and forward voltage, about 2 V for red and around 3 V for blue and white. LEDs need series resistors or current regulators to limit current. Efficient blue LEDs, developed in the early 1990s, earned the 2014 Nobel Prize in Physics and enabled white LED lighting. Photodiodes and solar cells reverse the process, turning light into current.
Other diode uses
Diodes protect circuits from reversed battery connections and voltage spikes (flyback and transient-voltage-suppressor diodes), clamp signals to safe ranges, combine power sources, and detect radio signals. Varactor diodes act as voltage-controlled capacitors for tuning. Diode logic and diode-based mixers appear in RF circuits.
Worked example
An LED with forward voltage 2.0 V should run at 15 mA from a 5 V supply. The series resistor must drop \(5 - 2 = 3\) V at 15 mA: \(R = \frac{3}{0.015} = 200\) \(\Omega\); the nearest E12 value, 220 \(\Omega\), gives about 13.6 mA. The resistor dissipates about \(3 \times 0.0136 \approx 0.04\) W, so a 0.25 W resistor is ample.
Key takeaways
Semiconductors such as silicon have controllable conductivity; doping creates n-type (electrons) and p-type (holes) material. A p-n junction forms a depletion region and conducts in forward bias but blocks in reverse: a diode. Silicon diodes drop about 0.6 to 0.7 V when conducting, following the exponential Shockley equation; Schottky diodes drop less. Half-wave and bridge rectifiers with smoothing capacitors convert AC to DC. Zener diodes regulate at their breakdown voltage. LEDs emit light, need current limiting, and blue LEDs won the 2014 Nobel Prize; photodiodes and solar cells convert light to current. Diodes also protect, clamp, and tune circuits.
Flashcard
Material with controllable conductivity
Check your understanding
Why must an LED connected to a 9 V battery have a series resistor?